68 



Experiment j}^o. 9. — This experiment may be varied bj covering 

 the soil in some of the pots with leaves or straw or paper, care being 

 taken tliat the added weight of the foreign matter be properly 

 accounted for. 



Soil Temperature. 



If a kernel of corn be placed in the ground in early spring before 

 the soil has become warm, the seed will not germinate. Abundance 

 of moisture and oxygen may be present, but the third requisite for 

 germination, proper temperature, is lacking. The soil is very slow 

 to become warm in the spring, and this is due to the large amount of 

 water which must be evaporated. During the winter and spring 

 the rain and melting snow liave saturated the soil. The under- 

 drainage is deficient and there is no way for the escape of the surplus 

 water except by evaporation, and evaporation is a cooling process. 

 A well-drained soil is thus warmer. 





■ v-'v,':.v::'5 



•ViV'S •.-• ;»^ 



- ■-./."vi".'*!.;.-- ' . .V!''.-: 





82. — The mcss-gro^cn lawn or grass plot. 



The atmosphere is much quicker to respond to changes in tem- 

 perature than is the soil. In the spring the air is warm while the 

 soil continues cold and the rains which fall during this time are 

 warmed by passing through the warm air, and in sinking through 

 the soil the water parts with some of its heat and the soil is made 

 warmer. During mid-summer the soil becomes very warm, and it is 

 not affected by cool nights, as is the atmosphere. Consequently a 

 summer rain may be several degrees cooler than the soil, and in 

 passing through the soil the water takes up some of the heat, and 

 the soil conditions are made more favorable for plant growth. 

 Therefore, soil temperature is regulated somewhat by the rainfall. 



Experiment No. 10. — The color of a soil also affects its tempera- 

 ture, a dark soil being warmer than a light-colored soil. By having 



450 



